The invention relates generally to wireless communications and, more particularly, to wireless communication system communication link optimization.
In communication systems it is typically desired to provide at least a threshold level of service quality while maximizing system capacity. For example, in point to multipoint wireless communication systems, such as those commonly used in providing cellular telephone and personal communication services (PCS), subscribers generally require that a service provider provide communications having at least a minimum level signal quality, such as link quality equivalent to plain old telephone service (POTS), that service be available substantially all the time, e.g., limited number of blocked calls, and that communications be successfully completed, e.g., no dropped calls.
In wireless communication systems in particular communication quality and communication capacity often have an inverse relationship. For example, as communication capacity is increased, such as through more dense reuse of traffic channels, signal quality may be decreased, such as through each such traffic channel experiencing higher levels of interference energy. Accordingly, service providers must often balance providing desired levels of communication capacity with service quality issues.
In code division multiple access (CDMA) networks, for example, a number of communication signals are allowed to operate over the same frequency band simultaneously. Each communication unit is assigned a distinct, pseudo-random, chip code which identifies signals associated with the communication unit. The communication units use this chip code to pseudo-randomly spread their transmitted signal over the allotted frequency band. Accordingly, signals may be communicated from each such unit over the same frequency band and a receiver may despread a desired signal associated with a particular communication unit. However, despreading of the desired communication unites signal results in the receiver not only receiving the energy of this desired signal, but also a portion of the energies of other communication units operating over the same frequency band. Accordingly, CDMA networks are interference limited, i.e., the number of communication units using the same frequency band, while maintaining an acceptable signal quality, is determined by the total energy level within the frequency band at the receiver.
In order to provide reasonable capacity in the CDMA network, transmitted signals are often power controlled to reduce energy transmitted within the CDMA frequency band while maintaining sufficient power to provide an acceptable signal at a receiving unit. Through intelligent power control, excess energy within the service area may be limited and, therefore, signal quality improved and/or capacity increased.
Accordingly, in a cellular communication network, a base transceiver station (BTS) or the like, may operate to receive information from mobile units with respect to a received signal as experienced by the mobile unit in order to make determinations as to the transmit power level to be utilized in communications with this particular mobile unit. This is an example of closed control loop operation.
However, a problem that has been experienced in implementing such a closed loop system is that, in order for the control loop to perform correctly, typically a relatively large amount of information must be communicated in the reverse link, e.g., a mobile unit provides substantial data in the reverse link in order for the BTS to adequately and accurately control transmit power levels. Moreover, generally speaking, the better the desired performance, e.g., more the transmit power is optimized for experienced conditions, the more information which is required to be fed back. It should be appreciated that, when providing feedback information for the closed control loop operation, the control system is occupying resources of the reverse link. In a system in which communications are substantially symmetric, such as is typical in two way voice communication, utilization of reverse link capacity for transmission of information not directly associated with subscriber payload is often a critical concern. Accordingly, it is often desirable to minimize the use of reverse link capacity for such control loop operation.
Further capacity and/or signal quality improvement may be provided in communication systems, such as the aforementioned point to multipoint wireless communication systems, through the use of directional antenna beams in the communication links. For example, adaptive array antennas may be utilized to provide enhanced signal quality through advanced xe2x80x9cbeam formingxe2x80x9d techniques as shown and described in the above referenced patent application entitled xe2x80x9cPractical Space-Time Radio Method for CDMA Communication Capacity Enhancement.xe2x80x9d For example, angle of arrival (AOA) information determined from a received signal at an adaptive array antenna may be utilized in accurately determining beam forming coefficients for use in the reverse link in order to provide improved capacity.
Although it is possible to estimate suitable forward link beam forming coefficients from corresponding reverse link beam forming coefficients, this solution does not always provide desired results. Specifically, this method relies upon a sufficiently high correlation between reverse link and forward link spatial structure. However, this is not always the case. For example, a difference in forward link and reverse link frequencies may result in there being insufficient correlation between the links. Additionally, the various multi-paths experienced in the two links may be significantly different.
A mobile unit might be adapted to accurately evaluate the forward link channel (spatial and temporal) in order to provide forward link channel information to a BTS for use in beam forming. However, there is a challenge in relaying this channel information to the serving BTS. Initially, it should be appreciated that current BTS infrastructure does not provide reverse link signaling adapted to communicate such information and, therefore, would require adaptation of the BTS systems as well as the mobile units. For example, the IS-95, GSM, and G3 interface standards provide very little reverse link capacity for such control signals. Moreover, as discussed above, it is desirable to minimize the use of reverse link capacity for control loop operation in order to provide desired subscriber payload capacity. Accordingly, the addition of forward link channel information to the aforementioned power control information, which itself is already consuming valuable reverse link capacity, is typically undesirable.
A need therefore exists in the art for systems and methods which allow for optimization of communication system capacity and signal quality without requiring an unacceptable amount of feedback information. A further need exists in the art for such systems and methods to provide forward link beam forming for optimized operation utilizing information with respect to evaluation of the forward link spatial channel by a receiving unit.
The present invention is directed to systems and methods which provide optimized communication links for communication conditions experienced by a plurality of subscriber units. For example, preferred embodiments of the present invention utilized in optimizing communication links in a cellular wireless communication system operate to control antenna beam width (e.g., antenna beam main lobe contour), length (e.g., antenna beam signal transmission power level), and/or direction (e.g., bore sight azimuthal and/or elevation orientation) selected and/or refined to thereby provide effective communications.
Although forward link characteristics may be optimized given enough feedback from mobile units, the amount of information required for accurate and adequate forward link channel optimization typically exceeds the reverse link capacity available for such uses. For example, link characteristics are often a function of signal fading (shadowing and Raleigh) which, in a highly mobile environment such as that of mobile cellular communications, require substantial amounts of information provided at a relatively rapid intervals in order to optimize a particular mobile unit""s communication links. Preferred embodiments of the present invention implement a control loop in a point to multipoint system which provides a suitable amount of information for link optimization of an entire population of forward link beams rather than, or in addition to, controlling individual forward link beams. Accordingly, a preferred embodiment of the present invention divides a service area associated with a point to multipoint system, such as a cell associated with a cellular communication system BTS, into a plurality of segments such that beam forming parameters are optimized with respect to these segments as mobile units operate therein.
In operation according to a preferred embodiment of the present invention, a cell is divided into a grid of segments. When a mobile unit operates in the cell the system determines the direction and distance of the mobile unit and, accordingly, determines a particular segment the mobile unit is operating within, retrieves a currently optimized forward link antenna beam for use with that segment, and illuminates the mobile unit with that antenna beam. As the mobile unit operates within that segment a control loop (also referred to herein as a segment control loop) is preferably operated to further optimize the segment""s forward link antenna beam. Preferably, when the mobile unit ceases to operate within that segment the control loop with respect to that mobile unit""s operation within the segment""s forward link antenna beam is stopped. The currently determined antenna beam configuration be stored in the configuration database for later retrieval and use, if desired. If the mobile unit appears within another segment, the process is repeated with respect to the antenna beam associated with that subsequent segment. Accordingly, for each segment in the grid, the preferred embodiment system provides a control loop which minimizes the forward link antenna beam as time progresses and the subscribers pass through the cell.
Embodiments of the present invention may be configured such that segments are defined in which a number of mobile units may be simultaneously operable therein. Accordingly, in a preferred embodiment, a segment control loop as described above is operated for optimization of the segment""s forward link antenna beam which utilizes information with respect to a plurality of mobile units operable within the segment. For example, information associated with all, or some subset, of the mobile units currently operable in the segment may be utilized, such as by averaging, weighted averaging, or other statistical combining, to thereby optimize the segment""s forward link antenna beam.
The control loops of the present invention may converge on the optimum antenna beam configurations over a period of days, weeks, and months. However, such a relative slow control loop is expected to be acceptable in many implementations, such as a cell site retrofit because the continuing time wise improvement in capacity and/or signal quality as the antenna beams are optimized over that of a typical prior art BTS implementation provide desired signal quality and/or capacity increase.
Link characteristics in a highly mobile environment are subject to change rapidly and may vary appreciably at different locations, even within a segment. Accordingly, segment control loops of the present invention are preferably adapted to provide optimization of communication links such that link conditions experienced by one mobile unit, or some minority of mobile units, do not undesirably affect optimization decisions. For example, a particular mobile unit may remain stationary in a particular segment for a very long time while substantially all other mobile units pass through the segment at a high rate of speed. If a control loop were to continuously utilize link characteristic information associated with this stationary mobile unit, the forward link of the segment would likely tend to converge to a link optimized for this particular mobile unit, which may result in a forward link which is less than optimum for the remaining mobile units. Therefore, preferred embodiment control loops are adapted to optimize a forward link antenna beam associated with the segment""s forward link without allowing a single or a few mobile units to undesirably affect the optimization results.
A preferred embodiment of the present invention recognizes the existence of a stationary, relatively slow moving, or otherwise anomalous mobile unit or subset of mobile units and alters utilization of information associated with that mobile unit or subset of mobile units in optimizing the forward link. For example, an embodiment of the present invention operates to cease utilization of information associated with that mobile unit or subset of mobile units in optimizing the segment forward link. A preferred embodiment of the present invention provides a control loop for that mobile unit or a subset of mobile units (also referred to herein as an individual control loop) which further optimizes a forward link for use with such a mobile unit or subset of mobile units. However, because this further optimized forward link is not expected to be representative of the forward link conditions experienced throughout the associated segment, a preferred embodiment of the present invention does not operate to update a segment optimized forward link database.
According to an alternative embodiment of the present invention, rather than providing the aforementioned individual control loop, the segment control loop may operate to continue to use information associated even with a stationary, relatively slow moving, or otherwise anomalous mobile unit, but is adapted to mitigate undesired affects resulting therefrom. For example, information associated with the various mobile units may be weighted such that operation of the anomalous mobile units does not result in an undesired segment forward link.
Preferably, communication link parameters are optimized according to the present invention utilizing forward link channel information, such as frame error rate (FER), bit energy to noise ratio (Eb/No), angle spread, and/or like information, provided by a receiving unit disposed in a corresponding segment. According to a preferred embodiment, the forward link channel information utilized is information available at the mobile unit without specific adaptation according to the present invention, such as the aforementioned FER or Eb/No information as may be used in providing prior art power control. The use of such information is preferred as such information may already be provided in an existing closed control loop and, therefore, not require the use of additional reverse link capacity in implementing the present invention. Additionally or alternatively, embodiments of the present invention may utilize forward link channel information which requires adaptation of mobile units in order to estimate, such as the aforementioned angle spread. Preferably, such information is selected such that it may be carried in an existing control loop without substantial alteration thereof and/or without requiring a substantial increase in control loop use of reverse link capacity.
To reduce the impact upon reverse link capacity while still providing information sufficient for an accurate understanding of the forward link channel characteristics, the preferred embodiment of the invention optimizes the beam forming parameters over a significant amount of time. For example, embodiments of the present invention operate to optimize beam forming parameters throughout the operation of an associated BTS.
Preferred embodiments of the present invention are adapted to provide optimization of communication links in response to a plurality of experienced phenomena. For example, operation of the present invention may provide optimization of an antenna beam such that the antenna beam is narrowed from an initial antenna beam configuration to a minimum beam width suitable for serving a particular segment. Thereafter, operation of the present invention may provide optimization of this antenna beam such that the antenna beam is widened and/or the antenna beam direction changed in order to adjust for an environmental change, such as the construction of a building in an area between the BTS and the particular segment.
A technical advantage of the present invention is provided in that the amount of information feedback utilized in implementing a control loop, preferably adjusted and controlled over a very long period of time, according to the invention is minimized. A further technical advantage of the present invention is provided in that the control loop feed back utilized according to preferred embodiments utilizes control loop information which is already available, thereby minimizing alteration of standard system interfaces, system equipment, and the like in order to implement the present invention.
A technical advantage of the present invention is provided in the optimization of a forward link for segments based upon information provided by a plurality of receiving units operable therein over time using a segment control loop. A still further technical advantage is provided in the optimization of a forward link for individual receiving units which have operating characteristics suitable for optimization without undesirably burdening a reverse link using an individual control loop over time.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.